1. Field of the Invention
This invention relates generally to rocket motors and more particularly to regeneratively cooled rocket combustors.
2. Description of the Prior Art
Regenerative cooling in liquid-propellant rocket combustors is a widely applied method for improving the capability of their combustor walls to withstand high combustion temperatures and heat transfer rates. The method commonly feeds one or possibly both of the propellants through channels in the thrust chamber wall for cooling purposes before they are injected into the combustion chamber. In one generally accepted design, these channels are formed by constructing an inner liner from a collection of mutually adjoined tubes to form a tube wall combustor. In another design, a series of spaced apart, longitudinally extending grooves are machined into the exterior surface of the inner combustor liner whereupon an outer liner is formed thereover. As a result of the latter technique, the combustor presents smooth inner walls for defining the interior of the combustion chamber and the coolant channels are wholly enclosed within the combustor wall. This latter design is generally preferred over the tube-wall construction although for purposes of this invention they are equivalent. The latter design is also currently employed in the combustor of the Space Shuttle Main Engine (SSME).
In engines such as the SSME, the coolant channels are generally rectangular, with the long axis of the rectangle oriented radially with respect to the axis of the combustor. It has been found that the characteristics of the heat flux through the hot-side surface of the combustor wall renders the most radially outwardly portions of the cooling passages significantly less effective in transferring heat from the combustor wall than the more radially inward portions. This situation, in turn, requires that the hot-side combustor wall be configured to maximize the heat transfer rate at the more radially inwardly portions of the coolant channels. This requirement is especially important in engines of advanced design wherein improved performance is to be effected by increased combustion temperatures and combustion pressures.
When the cooling problem is not properly checked, the roofs of the coolant channels become overheated and begin bowing outwardly from the coolant channels under the force of pressure of the coolant fluid therein. Upon bowing, the hot-side surface of the roofs extend beyond the boundary layer of gases on the hot-side surface of the combustor to become exposed to the rapidly passing combustor gases being ejected from the combustor. As a result, the exposed portions of the affected channel roofs suffer scarfing which rapidly abrades away the exposed portions of the roofs. As portions are eroded away, the roof becomes increasingly weaker and bows more and more until finally the roof breaches. Upon such failure, the coolant channel will leak coolant fluid into the combustion chamber, thereby depriving upstream sections of the combustion adequate coolant flow. If the leaks are severe and/or if they occur in a sufficient number of coolant passages, a horrendous, explosion-type combustor failure will likely occur.
Heretofore, designers have been intent upon trying to overcome the heat flux problem by either increasing the number of coolant channels, or by minimizing the thickness of the portions of the combustor wall between the hot-side surface of the combustor and the coolant channels (hereinafter referred to as the roofs of the coolant channels) or by making the channels as narrow as possible so that the span of the roofs are decreased. All these solutions however have serious shortcomings. If the roofs are made too thin they cannot contain the tremendous pressure of the coolant fluid. If the channels are made too numerous and/or too narrow, the channels take on a highly rectangular shape which severely inhibits the flow of coolant fluid through the channels with the result that, friction within the coolant channels is significantly increased and circulation of cooling fluid at the most radially inward portions of the coolant chanels is hampered.